A conventional scanner processing head for fast spatial processing of workpieces using laser beams consists of scanner optics usually including two mirrors that deflect a collimated or almost collimated beam in a first and a second spatial direction and guide it onto the workpiece using a plane field objective. In an alternative system, downstream of the focusing lens, a focused beam is guided by at least one scanner mirror onto the workpiece. For processing three-dimensional objects using such a scanner optics, the position of the focal point of the laser radiation in the beam direction must additionally be adjustable. This necessitates a device for focusing the laser beam that permits displacement of the focal position in the beam direction.
If the laser connected to the scanner optics is a solid-state laser, the output laser beam is preferably transmitted to the scanner optics through an optical fiber (laser light guide). In the scanner optics, the laser beam emerges from the laser light guide in a diverging manner, is collimated using a collimation lens, is subsequently deflected by the scanner mirrors and is focused onto the workpiece using a plane field objective. To adjust the focus along the beam axis, the divergence of the beam impinging on the focusing objective is changed by, for example, moving the collimation lens along the beam axis.
One disadvantage of this approach is that the collimating lens is relatively large and therefore heavy, and the displacement of the lens must be relatively large to obtain a noticeable focus shift of the working beam. With this approach, the usually required high dynamics of the shifting motion involves a very high driving power, requiring a large and heavy construction (with even increased moving mass) and complex control and cooling of the actuators. Moreover, the large and highly accelerated mass exerts a considerable reaction force on the static part of the assembly, which may cause undesired motion or oscillations of the beam guiding system (which is mounted for example, to a robot arm) that can be compensated for or minimized only with considerable additional expense.
The documents JP05002146, JP2000197984, JP07116869 and JP2003012346 moreover disclose the use of a telescope system having two lenses that may have different sizes, to influence the divergence of the beam. One of the two lenses is displaced. Typically, the smaller lens that can be displaced with less force to change the separation between the two lenses is the lens that is displaced. These lenses are situated in the collimated beam and not in the divergent beam.